We're not talking about a few dead fish littering your local beach. Mass die-offs are individual events that kill at least a billion animals, wipe out over 90 percent of a population, or destroy 700 million tons—the equivalent weight of roughly 1,900 Empire State Buildings—worth of animals.

And according to new research, such die-offs are on the rise.

Thestudy, published this week in the Proceedings of the National Academy of Sciences, is the first to examine whether mass die-offs have increased over time.

Researchers reviewed historical records of 727 mass die-offs from 1940 to 2012 and found that over that time, these events have become more common for birds, marine invertebrates, and fish. The numbers remained unchanged for mammals and decreased for amphibians and reptiles. (See "What's Killing Bottlenose Dolphins? Experts Discover Cause.")

Disease, human-caused disturbances, and biotoxins—like the red tides caused by algae that are prevalent along American coastlines—are three major culprits.

It's unclear what's making diseases more common or why red tides happen more often. Climate change and environmental degradation are some contenders.

Researchers also don't know why die-off rates differ between animal groups. The scientists may have missed some die-off events, giving the impression of steady or declining rates in mammals, amphibians, and reptiles. Or, these groups may not be as affected as fish, birds, and marine invertebrates.

What's Next

What's clear is that lack of coordinated attention from scientists is a problem, the study authors say. In fact, "at this time, the vast majority of [mass die-offs] are presented in newspapers," they write.

There needs to be better monitoring of these events, they say, since that's the only way we'll know how much trouble life on Earth is in.